Exhaust aftertreatment systems receive and treat exhaust gas generated by an internal combustion engine. Typical exhaust aftertreatment systems include any of various components configured to reduce the level of harmful exhaust emissions present in the exhaust gas. For example, some exhaust aftertreatment systems for diesel powered internal combustion engines include various components, such as a diesel oxidation catalyst (DOC), particulate matter filter or diesel particulate filter (DPF), and a selective catalytic reduction (SCR) catalyst. In some exhaust aftertreatment systems, exhaust gas first passes through the diesel oxidation catalyst, then passes through the diesel particulate filter, and subsequently passes through the SCR catalyst.
Each of the DOC, DPF, and SCR catalyst components is configured to perform a particular exhaust emissions treatment operation on the exhaust gas passing through the components. Generally, the DOC reduces the amount of carbon monoxide and hydrocarbons present in the exhaust gas via oxidation techniques. The DPF filters harmful diesel particulate matter and soot present in the exhaust gas. Finally, the SCR catalyst reduces the amount of nitrogen oxides (NOx) present in the exhaust gas.
The SCR catalyst is configured to reduce NOx into less harmful emissions, such as N2 and H2O, in the presence of ammonia (NH3). Because ammonia is not a natural byproduct of the combustion process, it must be artificially introduced into the exhaust gas prior to the exhaust gas entering the SCR catalyst. Typically, ammonia is not directly injected into the exhaust gas due to safety considerations associated with the storage of gaseous ammonia. Accordingly, conventional systems are designed to inject a diesel exhaust fluid or reductant into the exhaust gas, which is capable of decomposing into gaseous ammonia in the presence of exhaust gas under certain conditions. The reductant commonly used by conventional exhaust aftertreatment systems is a urea-water solution (hereinafter “urea”).
Generally, the decomposition of urea into gaseous ammonia occupies three stages. First, urea evaporates or mixes with exhaust gas. Second, the temperature of the exhaust causes a thermolysis-induced phase change in the urea and decomposition of the urea into isocyanic acid (HNCO) and NH3. Third, the isocyanic acid reacts with water in a hydrolysis process under specific pressure and temperature concentrations to decompose into ammonia and carbon dioxide (CO2). The gaseous ammonia is then introduced at the inlet face of the SCR catalyst, flows through the catalyst, and is consumed in the NOx reduction process. Any unconsumed ammonia exiting the SCR system can be reduced to N2 and other less harmful or less noxious components using an ammonia oxidation catalyst.
SCR systems typically include a urea source and a urea injector or doser coupled to the source and positioned upstream of the SCR catalyst. The urea injector injects urea into a decomposition space through which an exhaust gas stream flows. Upon injection into the exhaust gas stream, the injected urea spray is heated by the exhaust gas stream to trigger the decomposition of urea into ammonia. As the urea and exhaust gas mixture flows through the decomposition space, the urea further mixes with the exhaust gas before entering an the SCR catalyst. Ideally, urea is sufficiently decomposed and mixed with the exhaust gas prior to entering the SCR catalyst to provide an adequately uniform distribution of ammonia at the inlet face of the SCR catalyst.
Some prior art exhaust aftertreatment systems, however, do not provide adequate decomposition and mixing of injected urea. Often, conventional systems cause exhaust gas recirculation or low temperature regions within the decomposition space. Exhaust gas recirculation and low temperature regions may result in inadequate mixing or decomposition, which may lead to the formation of solid urea deposits on the inner walls of the decomposition space and urea injector. Additionally, inadequate mixing may result in a low ammonia vapor uniformity index, which can lead to uneven distribution of the ammonia across the SCR catalyst surface, lower NOx conversion efficiency, and other shortcomings.
The formation of solid urea deposits and uneven ammonia distribution may also be caused by urea spray being deflected away from an intended target. Following injection, the urea spray typically rapidly decelerates due to entrainment of exhaust gas into the spray. Rapid deceleration reduces urea spray penetration and momentum, which makes the injected urea spray susceptible to substantial redirection when contacted by exhaust flow gases. Undesirable redirection of urea spray may result in urea spray unintentionally contacting certain surfaces of the decomposition space (e.g., an inner wall of a decomposition tube and an upper portion of a mixer) and forming solid urea deposits thereon. The formation of solid urea deposits within the decomposition space typically results in a lower amount of ammonia concentration and a lower ammonia distribution uniformity index at the inlet face of the SCR catalyst, which can degrade the performance and control of the SCR catalyst.